1. Field of the Invention
The present invention relates in general to a molding clamping apparatus, and more particularly to such a molding clamping apparatus which includes a ballscrew shaft and a ballscrew nut, for moving a movable plate relative to a stationary plate, so as to open, close and clamp a mold consisting of movable and stationary mold halves that are to be fixed to the movable and stationary plates, respectively.
2. Discussion of Related Art
As an apparatus for clamping a mold used for an injection molding machine, a press forming machine or other forming machine, for example, there is known a mold clamping apparatus equipped with a hydraulic cylinder which directly transmits a driving force to the movable plate, or which indirectly transmits the driving force to the movable plate through a toggle link mechanism. There is also known a mold clamping apparatus equipped with a pair or pairs of ballscrew shaft and ballscrew nut which serves to convert a rotational driving force to a reciprocal driving force and then transmit the reciprocal driving force to the movable plate.
As examples of the mold clamping apparatus equipped with the ballscrew shafts and nuts, JP-Y2-01-36587 and JP-Y2-01-36588 disclose apparatuses, in each of which four tie bars are provided to extend between the stationary and movable plates and have respective external threads formed in their outer circumferential surfaces, so as to serve the ballscrew shafts. The external threads of the four tie bars engage respective four internally-threaded ballscrew nuts which are attached to the movable plate. In the apparatus disclosed in JP-Y2-01-36587, the four tie bar are adapted to be rotatable about their respective axes in the forward and reverse directions, while the four ballscrew nuts are fixed to the movable plate so as to be not rotatable relative to the movable plate, so that the four ballscrew nuts are axially moved with rotations of the four tie bars which are driven by an electric motor, whereby the movable plate carried by the four ballscrew nuts are moved toward and away from the stationary plate. In the apparatus disclosed in JP-Y2-01-36588, the four ballscrew nuts are supported by the movable plate rotatably relative to the movable plate, and are driven by an electric motor so as to be rotated, so that the four ballscrew nuts are axially moved, whereby the movable plate carried by the four ballscrew nuts are moved toward and away from the stationary plate.
Owing to the construction in which the four tie bars extending between the movable and stationary plates serve as the ballscrew shafts, each of the above-described apparatuses advantageously has smaller total length and size than those of an apparatus in which the ballscrew shafts are disposed in one of opposite sides of the movable plate remote from the stationary plate. However, the above-described apparatuses, in which the rotational driving force generated by the electric motor is transmitted to the tie bars or the ballscrew nuts through gears, suffer from the problem that the gears meshing with each other generate large noises. This problem of the noises might be somewhat resolved by employing a timing belts or other belts in place of the gears. However, the belts produces particles or dusts as the belts are worn, and the produced particles or dusts problematically contaminate the formed product and the environment.
In the apparatus disclosed in JP-Y2-01-36588, each tie bar is fixed, at one of axially opposite end portions thereof remote from the stationary plate, to an end plate (designated by the reference numeral 14 in FIG. 1 of the publication) which is disposed movably in the axial direction of the tie bars, so that the end plate is moved when the tie bars are elongated by a reaction force of a mold clamping force (compressive force) which is generated by and between the movable and stationary plates in an operation for closing the mold. That is, the end plate is displaced by a distance corresponding to an amount of the elongation of the tie bars, whereby the servo motor is protected from application of an excessively large load thereto due to the elongation of the tie bars, or whereby a load applied to the servo motor is reduced when the tie bar is elongated. However, the arrangement enabling the displacement of the end plate requires a complicated structure, thereby inevitably complicating even the construction of the entirety of the apparatus.
In view of the above-described problems, the present applicant has proposed a mold clamping apparatus in Japanese Patent Application No. 11-211600. In the proposed apparatus, each of the ballscrew shafts is fixed at an axial end portion thereof to the movable plate, and extends from the movable plate toward the stationary plate so as to pass through the stationary plate, such that the other axial end portion of each ballscrew shaft projects from one of opposite side faces of the stationary plate remote from the movable plate, over a predetermined distance. The servo motors are fixed to the corresponding portions of the one of the opposite side faces of the stationary plate, from which the other axial end portion of each ballscrew shaft projects. Each ballscrew shaft engages, at the other axial end portion projecting from stationary plate, the corresponding one of the ballscrew nuts which are directly fixed to the rotors of the respective servo motors.
This proposed mold clamping apparatus, in which the ballscrew nuts are fixed directly to the rotors of the respective servo motors, no longer suffers from the above-described problem encountered in the conventional apparatus in which the rotational driving force generated by the servo motor is transmitted to the ballscrew shafts and nuts via the gears or belts. That is, the proposed mold clamping apparatus suffers from neither noises generated by the mutually meshing gears, nor dusts produced due to wear of the belts.
However, in the proposed mold clamping apparatus, in which the ballscrew nuts are fixed to the rotors of the respective servo motors so as to be rotated together with the rotors, a relatively large rotational inertia acts on each servo motor, and an accordingly large load is applied to each servo motor, for example, upon initiation and termination of rotation of the servo motor, thereby causing some delay of each change in the momentum of the movable plate during an operation for opening and closing the mold, and increasing a time required for actual initiation or termination of the motion of the movable plate in response to a command inputted by the operator for initiating or terminating the motion of the movable plate. In this respect, the proposed mold clamping apparatus has a difficulty in performing an operation in which the product is required to be formed in a short time.
Further, in the proposed mold clamping apparatus, the servo motors are attached to the one of opposite side faces of the stationary plate remote from the movable plate, and the ballscrew shafts extends from the movable plate toward the stationary plate so as to pass through the stationary plate, as described above. In this arrangement, the above-described other axial end portion of each ballscrew shaft further projects from the stationary plate in a direction away from the movable plate, as the movable plate is moved toward the stationary plate. This arrangement deteriorates the condition of the operator""s operation, reducing the efficiency of the operation. For example, where this mold clamping apparatus is used for an injection molding machine, the projecting axially end portions of the ballscrew shafts, as well as the servo motors attached to the side face of the stationary plate, disturb various operations which should be carried out on the side of the stationary plate remote from the movable plate, such as an operation for removing a purging resin from a nozzle of the injection molding machine which is located on the side of the stationary plate remote from the movable plate.
Thus, even the apparatuses having the excellent features should be further improved to overcome the drawbacks addressed above.
It is therefore a principal object of the present invention to provide a mold clamping apparatus having an improved construction which prevents generation of noises and production of dusts during operation of the apparatus, and which reduces a rotational inertia acting on each servo motor that generates the driving force for opening, closing and clamping the mold, and accordingly reduces a load applied to the servo motor upon initiation and termination of rotation of the servo motor, so as to shorten the time required for forming the product.
It is a first optional object of the invention to provide a mold clamping apparatus having a novel construction in which the servo motors and the ballscrew shafts are disposed in a manner that facilitates the operator""s operation on a machine incorporating the mold clamping apparatus, and also other operations required for forming a desired product.
It is a second optional object of the invention to provide a mold clamping apparatus which is simple in construction and is capable of preventing application of an excessively large load to the servo motors when the ballscrew shafts are elongated by a reaction force of a mold clamping force generated in an operation for closing the mold.
The above objects may be achieved according to the principle of the present invention, which provides a mold clamping apparatus for clamping a mold consisting of a stationary mold half and a movable mold half, the mold clamping apparatus comprising: a stationary plate to which the stationary mold half is to be fixed; a movable plate to which the movable mold half is to be fixed and which is disposed so as to be opposed to the stationary plate in a predetermined direction; and a moving device which moves the movable plate toward and away from the stationary plate in the predetermined direction, so as to close and open the mold, wherein the moving device comprises:
(a) a plurality of ballscrew shafts each of which extends in the predetermined direction from the stationary plate toward the movable plate, and each of which is supported at one of axially opposite end portions thereof by the stationary plate, so as to be rotatable relative to the stationary plate and so as not to be displaceable relative to the stationary plate in an axial direction thereof; (b) a plurality of servo motors which drive the respective ballscrew shafts, and which are disposed on one of opposite sides of the movable plate remote from the stationary plate, each of the servo motors having a rotor which is connected to the other of the axially opposite end portions of the corresponding one of the ballscrew shafts such that the rotor and the corresponding ballscrew shaft are rotatable together; and (c) a plurality of ballscrew nuts which engage the respective ballscrew shafts and are fixed to the movable plate, so as to be moved in the axial direction by rotations of the respective ballscrew shafts, for thereby moving the movable plate toward and away from the stationary plate in the axial direction.
In the mold clamping apparatus constructed according to the principle of the invention, the ballscrew shafts each having a diameter smaller than that of each of the ballscrew nuts are directly connected to the rotors of the respective servo motors such that each ballscrew shaft is rotatable in the forward and reverse directions, together with the rotor of the corresponding one of the servo motors. This construction provides a smaller rotational inertia acting on the rotor of each servo motor, than where the ballscrew nuts are directly connected to the rotors of the servo motors, and accordingly reduces a load applied to each servo motor upon initiation and termination of rotation of the servo motor, thereby shortening a time required for actual initiation or termination of the motion of the movable plate in response to a command inputted by the operator for initiating or terminating the motion of the movable plate during an operation for opening and closing the mold, resulting in a reduced time required for forming the product. Further, since the present mold clamping apparatus does not have gears, belts or any other members for transmitting a rotational driving force generated by each servo motor, to the ballscrew shaft or nut, it is possible to prevent generation of noises due to the mutually meshing gears and also production of dusts due to wear of the belts during operation of the apparatus.
In the present mold clamping apparatus, the servo motors are located on the side of the movable plate remote from the stationary plate, as described above. Thus, even where this mold clamping apparatus is used to be incorporated in an injection molding machine, for example, the servo motors do not disturb operations which should be carried out on the side of the stationary plate remote from the movable plate, such as an operation for taking out a purging resin from a nozzle of the injection molding machine which is located on the side of the stationary plate remote from the movable plate.
In the present mold clamping apparatus, the ballscrew shafts extend from the stationary plate toward the movable plate in parallel with the direction in which the movable plate is moved by the moving device, and each of the ballscrew shafts is supported at one of the axially opposite end portions by the stationary plate, so as not to be movable relative to the stationary plate in the axial direction. With rotation of the respective ballscrew shafts, the ballscrew nuts fixed to the movable plate are moved in the axial direction, for thereby moving the movable plate toward and away from the stationary plate in the axial direction. Thus, in contrast to a conventional apparatus, in which the ballscrew shafts are axially moved so as to move axially the movable plate so that the axial end portions of the respective ballscrew shafts project axially outwardly from the stationary plate, the ballscrew shafts do not have to be axially moved so as to axially move the movable plate, thereby not disturbing the operations which are carried out on the side of the stationary plate remote from the movable plate.
As is clear form the above description, the mold clamping apparatus of the present invention is capable of reduce the time required for forming the product while preventing generation of noises and production of dusts during operation of the apparatus. Thus, the present mold clamping apparatus can be used, for example, in an operation carried out in a clean room, an operation in which the product is required to be formed in a reduced time, and other kinds of operations for other purposes. Further, in the present mold clamping apparatus, the servo motors and the ballscrew shafts are disposed in a manner that facilitates the operator""s operation on a machine incorporating the mold clamping apparatus, and also other operations required for forming a desired product. It is noted that the moving device, which moves the movable plate relative to the stationary plate, serves to generate a mold clamping force for clamping the mold when each ballscrew shaft is rotated in the direction making the movable plate move toward the stationary plate even after the stationary and movable mold halves have been brought into contact with each other. In this sense, the moving device may be also referred to as an clamping-force generating device.
According to a first preferred form of the mold clamping apparatus of the invention, the rotor has, at a center thereof, an axial hole in which the other of the axially opposite end portions of the ballscrew shaft is received, and wherein the rotor has an internal spline, and the ballscrew shaft has, in the other of the axially opposite end portions, an external spline which engages with the internal spline of the rotor for enabling the ballscrew shaft to rotate together with the rotor and permitting a sliding movement of the ballscrew shaft relative to the rotor in the axial direction.
In the first preferred form of the mold clamping apparatus of the invention, each of the ballscrew shafts and the rotor of the corresponding servo motor are reliably connected to each other with the ballscrew shaft and the rotor being rotatable together with each other, by simply engaging the external and internal splines each other.
The external and internal splines mate with each other for inhibiting rotation of the ballscrew shaft relative to the rotor, namely, inhibiting displacement of the ballscrew shaft relative to the rotor in the circumferential direction, but permitting sliding movement of the ballscrew shaft relative to the rotor in the axial direction, namely, permitting displacement of the ballscrew shaft relative to the rotor in the axial direction. Accordingly, the external spline formed in the ballscrew shaft is axially slidable relative to the internal spline formed in the rotor, when the ballscrew shaft is elongated by a reaction force of a mold clamping force (compressive force) generated by and between the movable and stationary plates in an operation for closing the mold. That is, the external spline is axially displaced by a distance corresponding to an amount of the elongation of the ballscrew shaft, whereby the servo motor is protected from application of an excessively large load thereto due to the elongation of the ballscrew shaft, or whereby a load applied to the servo motor is reduced when the ballscrew shaft is elongated.
According to one advantageous arrangement of the first preferred form of the invention, the internal spline consists of a plurality of projections or grooves formed in an inner circumferential surface of the axial hole of the rotor and circumferentially spaced apart from each other, and the external spline consists of a plurality of grooves or projections formed in an outer circumferential surface of the other of the axially opposite end portions of the ballscrew shaft and circumferentially spaced apart from each other, and wherein the axial hole has a bottom face which cooperates with an axial end face of the other of the axially opposite end portions of the ballscrew shaft to define an axial spacing for permitting displacement of the ballscrew shaft relative to the rotor in the axial direction while the internal spline and the external spline are held in meshing engagement with each other.
According to a second preferred form of the invention, the mold clamping apparatus further comprises a tubular covering member which is disposed between the stationary plate and the movable plate and which surrounds at least a threaded part of each of the ballscrew shafts which portion has a thread formed therein. The tubular covering member protects the thread of each ballscrew shaft from being damaged. For example, in an operation for mounting the mold halves onto the stationary and movable plates, the covering member prevents the thread from being damaged due to contact of the thread with the mold halves. Further, the covering member prevents splashing of oil, grease and other lubricants which are applied to the ballscrew shaft.
According to a third preferred form of the invention, the mold clamping apparatus further comprises an actual-rotary-position detecting device which detects an actual rotary position of the rotor of each of the servo motors, a target-rotary-position setting device which sets a target rotary position of the rotor of each of the servo motors which makes the movable plate parallel to the stationary plate, and a control device which controls the servo motors such that the detected rotary position of the rotor of each of the servo motors coincides with the set target rotary position of the rotor of each of the servo motors.
The arrangement according to this third preferred form of the invention assures a high degree of parallelism of the stationary plate with respect to the movable plate which is carried by the plurality of ballscrew nuts, namely, which receives, in respective portions thereof, forces that are applied from the respective ballscrew nuts and that force the movable plate toward the stationary plate. Thus, the stationary and movable mold halves attached to the respective stationary and movable plates can be constantly brought into close contact with each other over the entirety of each of their mutually opposed surfaces with high stability, thereby assuring high quality of the formed product without a risk of forming a defective product.
Where this mold clamping apparatus having the above advantageous feature is used to be incorporated in an injection molding machine, it is possible to advantageously practice an injection molding according to the so-called injection compression forming method, in which a molten resin is compressed by a predetermined amount of force after the molten resin has been injected into a mold cavity in the mold, since the stationary and movable mold halves cooperating with each other to constitute the mold can be constantly brought into close contact with each other over the entirety of each of their mutually opposed surfaces with high stability, as described above.
According to a fourth preferred form of the mold clamping apparatus of the invention, the actual-rotary-position detecting device includes a rotary encoder which is built in each of the servo motors. Since each ballscrew shaft is directly connected in its axial end portion to the rotor of the corresponding servo motor, it is possible to use the built-in type rotary encoder which is built in the servo motor. Thus, the mold clamping apparatus can be made more simple in construction and compact in size, than where the rotary encoder as the actual-rotary-position detecting device is provided independently of the servo motor.
According to a fifth preferred form of the mold clamping apparatus of the invention, at least one of the ballscrew shafts consists of a right-hand ballscrew shaft, while each of the other of the ballscrew shafts consists of a left-hand ballscrew shaft.
It is considered possible that the each ballscrew shaft might be partially deformed to be distorted or twisted in a direction of the rotation of the ballscrew nut, for example, due to friction acting between the ballscrew shaft and nut. In the mold clamping apparatus according to this fifth preferred form of the invention, when the ballscrew shafts are thus twisted, the right-hand ballscrew shaft and the left-hand ballscrew shaft are twisted in the respective directions opposite to each other, thereby preventing rotation of the movable plate or displacement of the movable plate in the circumferential direction relative to the stationary plate, due to the twisting deformations of the respective ballscrew shafts. Thus, the arrangement of the present fifth preferred form is effective to prevent undesirable offset of the movable mold half relative to the stationary mold half in the circumferential direction, assuring reliable positioning of the movable mold half with respect to the stationary mold half in a direction perpendicular to the axial direction of the ballscrew shafts.
According to one advantageous arrangement of the fifth preferred form of the mold clamping apparatus of the invention, the ballscrew shafts consists of an even number of ballscrew shafts consisting of at least one pair of ballscrew shafts each of which consists of the right-hand ballscrew shaft and the left-hand ballscrew shaft.
According to a sixth preferred form of the invention, the mold clamping apparatus further comprises a guide way which extends in the predetermined direction and which supports the movable plate such that the movable plate is sidable on said guide way in the predetermined direction.
The arrangement of this sixth preferred form of the invention, as well as the above-described arrangement of the fifth preferred form, is effective to prevent undesirable offset of the movable mold half relative to the stationary mold half in the direction perpendicular to the axial direction, assuring reliable positioning of the movable mold half with respect to the stationary mold half with high accuracy.
According to a seventh preferred form of the invention, the mold clamping apparatus further comprises a load detecting device which detects amounts of loads acting between the respective ballscrew nuts and the movable plate when the mold is being clamped after the stationary and movable mold halves have been brought into contact with each other by the rotation of the respective ballscrew shafts, and an adjusting device which adjusts amounts of the rotations of the respective ballscrew shafts in a manner that equalizes the amounts of the loads detected by the load detecting device, to each other. The load detecting device may include a plurality of load cells which are interposed between the respective ballscrew nuts and the movable plate and which detect the amounts of loads acting between the respective ballscrew nuts and the movable plate.
In the seventh preferred form of the mold clamping apparatus of the invention, the adjusting device adjusts the amounts of the rotations of the respective ballscrew shafts, for example, by adjusting operating amounts of the respective servo motors which rotate the respective ballscrew shafts, such that the detected amounts of the loads are equalized to each other while the mold is being clamped. This arrangement makes it possible to equalize local forces which are applied to respective portions of the movable plate from the respective ballscrew nuts and which force the movable plate toward the stationary plate, so that a required clamping force consisting of the sum of the local forces is distributed evenly over the entirety of each of mutually opposed surfaces of the movable and stationary mold halves which are attached to the respective movable and stationary plates.
Accordingly, the movable and stationary mold halves are brought into mutually contact and forced toward each other with the clamping force being distributed evenly over the entirety of each of the mutually opposed surfaces of the mold halves. The even distribution of the clamping force prevents or minimizes undesirable appearance of molding fin or flash in a parting line portion of the product which is formed in the cavity defined by and between the movable and stationary mold halves. Further, in the seventh preferred form of the mold clamping apparatus, the mold opening and closing motions are carried out while the parallel relationship between the movable and stationary plates being well maintained, avoiding a risk of formation of scratch on the mutually opposed surfaces of the respective movable and stationary mold halves, thereby preventing the mold halves and the formed product from being damaged or broken during the mold opening and closing motions.